Micro-mechanical angular rate sensors (gyroscopes) are known in the art. They have been fabricated for some time on silicon wafers using conventional etching techniques. They can include a first element (primary oscillator, drive element) arranged in an x-y plane which is excited to perform an oscillatory motion, and a second element (secondary oscillator, output element) used for detection, which is connected with the first element via connecting elements. When the sensor performs a rotation motion Ω about an axis perpendicular to the oscillation axis, the Coriolis force of 2 m vr×u acts on the various masses of the body performing the rotation oscillation; this force is transferred by suitable measures to the detection element so as to deflect the detection element out of the x-y plane. This rotation motion is then detected by suitable means, e.g., by capacitive electrodes.
The aforedescribed system has been implemented to date in two different embodiments. EP 0906557 B1 describes an angular rate sensor with decoupled orthogonal primary and secondary oscillations. The primary oscillator is attached via a primary oscillator support in the center on the substrate and supports via torsion springs a secondary oscillator located in the same plane, wherein the torsion springs rigidly transfer the induced oscillation of the primary oscillator to the secondary oscillator. When the sensor rotates about a plane perpendicular to the plane in which the two oscillating elements are located, the Coriolis force affects both elements. While the secondary oscillator is then tilted out of its plane, the primary oscillator remains in this plane, because it is, on one hand, anchored on the substrate, making it nearly impossible for the primary oscillator to tilt out of this plane and, on the other hand, the torsion springs prevent the Coriolis force acting on the secondary oscillator to be coupled back to the primary oscillator.
The approach disclosed in U.S. Pat. No. 5,955,668 goes just the opposite way: the oscillating element, which is excited to perform a radial oscillation, encircles a tiltable sensor element which is attached to a substrate by way of two anchors. Torsion springs connect the oscillating element with the sensor element and are configured to neither transfer the oscillation of the oscillating element to the sensor element nor to couple the tilting motion of the sensor element produced by the Coriolis force back to the oscillating element.
The two aforementioned angular rate sensors are configured, as mentioned above, so as to be capable of detecting rotation motion perpendicular to the axis of the excited rotation oscillation. The rotation motion is hereby always detected through a deflection of the detection element out of the plane of the excitation oscillation; one side of the detection element hereby tilts towards the substrate on which the structure consisting of oscillation and detection element is anchored. Any gas molecules trapped between the substrate and the detection element can cause attenuation which distorts the result. These angular rate sensors must therefore be sealed in an evacuated housing.
The oscillating modes of oscillating element and detection element of the aforementioned angular rate sensors are not decoupled in the oscillation plane of the oscillating element. They are therefore incapable of detecting a rotation motion about the same axis about which the excitation element oscillates.
Accordingly, a way should be found for measuring the force to be detected independent of the centrifugal force produced by the excitation oscillation. U.S. Pat. No. 6,308,567 B1 proposes an angular rate sensor where two pairs of opposing masses are excited so as to perform a rotation oscillation in mutually opposite directions. Due to this oscillation, all these masses experience a centrifugal force in a radially outward direction. If an external rotation motion (to be detected) is applied to the system, then the vector of the resulting Coriolis force points for one pair of masses in the outward radial direction and for the other pair of masses in the inward radial direction. The effective measured force of the one mass pair is then subtracted by a computation from that of the other mass pair, which eliminates the centrifugal force from the total equation, whereas the absolute values of the Coriolis force are added together with a positive sign.
To ensure symmetry when eliminating the centrifugal force in the computation, four well matched C/V converters must be employed. Also required is a circuit for performing the required computation to eliminate the centrifugal force. The necessary evaluation circuit requires considerable space, which directly impacts production costs.
It is an object of the present invention to provide an angular rate sensor which obviates this disadvantage.